Abstract When a phosphate-buffered saline (PBS) solution is exposed to atmospheric-pressure plasmas generated in air, hydrogen peroxide H 2 O 2 in the solution is known to be decomposed by hypochlorite O C l − , which is formed in the solution from reactions between chlorine ions C l − present in the PBS solution and plasma-generated reactive oxygen species. Global numerical simulations of liquid-phase chemical reactions were performed to identify the reaction mechanisms of H 2 O 2 decomposition by solving known liquid-phase chemical reactions self-consistently. It has been confirmed that the decomposition of H 2 O 2 is indeed mostly due to the presence of O C l − in the solution. However, this study has also found that, in the presence of abundant hydroxyl ( O H ) radicals, the most efficient H 2 O 2 decomposition pathway can be a series of reactions that we call a chlorine monoxide cycle, where O C l − first reacts with O H to generate chlorine monoxide C l O , which then decomposes H O C l , rather than O C l − directly decomposing H 2 O 2 . The chlorine monoxide cycle generates O H as one of its byproducts, so once this cycle is initiated, it continues until either C l O − or H 2 O 2 runs out, as long as none of the intermediates are scavenged by other reactions.